In a lighting circuit 1A, there are provided a DC power circuit 3′ for outputting DC voltages having positive and negative polarities and a DC-AC converting circuit 4A for converting the output voltages into AC voltages through a plurality of switching elements and for then supplying the AC voltages to a plurality of discharge lamps 6—1 and 6—2. In the case in which each discharge lamp is to be lighted up, a polarity fixing period is set to define the polarity of the voltage to be supplied from the DC-AC converting circuit 3′ to the discharge lamp before and after the discharge lamp is lighted up to be positive or negative. For this period, control is carried out such that a current flowing to the discharge lamp does not exceed a predetermined limit current value. Consequently, when a first discharge lamp has already been lighted up and a second discharge lamp is to be lighted up, it is possible to relieve the influence of a thermal stress applied to the first discharge lamp by the control of an output polarity for the discharge lamp.